1. Field of the Invention
This invention relates to a semiconductor device having a vertical transistor wherein a gate electrode and a gate insulating film are buried in a semiconductor substrate and a method for manufacturing same. The invention also relates to a solid-state image sensing device having a vertical transistor at a pixel portion.
2. Description of the Related Art
In solid-state image sensing devices, high degrees of integration of circuits have been sought for improving sensitivity and there have been proposed a variety of techniques of keeping the area of a photodiode.
For instance, there is proposed a structure wherein a trench is formed over a photodiode formed at a back side of a silicon substrate and a transfer gate is provided inside the trench thereby forming a vertical transistor (see, for example, Japanese Patent Laid-Open No. 2005-223084, hereinafter referred to as Patent Document 1).
The vertical transistor formed inside the trench includes an electrode material for gate electrode, which is buried in the trench covered with the gate insulating film. The side wall portion and bottom portion of the trench serve as a channel portion.
This structure is able to increase an area of a photodiode when compared with a structure wherein a photodiode and a transfer gate are individually formed at the surface side of a substrate, thereby leading to an improvement in sensitivity.
By the way, because the vertical transistor includes a shape at which an electric field concentration begins, such as a corner portion of a vertical hole, structural reliability is poor.
For the formation of a gate insulating film made of a silicon oxide film, usual practice is to oxidize the surface of a silicon layer by a thermal oxidation (dry oxidation) process using O2 gas and HCl gas or a thermal oxidation (wet oxidation) process using O2 gas, HCl gas and H2 gas.
In the thermal oxidation process, typical of which are these dry and wet oxidation processes, an oxidizing agent such as oxygen (O2), OH or the like is diffused into the oxide film by the thermal energy and reacts with silicon atoms at the interface with the silicon, thereby causing the oxidation process to be advanced.
The reaction between the oxygen being diffused and the silicon at the interface depends on the thermal energy and the Si density at the interface. At the (110) face that is higher in silicon density than the (100) face, the oxidation proceeds more rapidly. Hence, the thickness of the oxide film has a strong silicon face orientation dependence.